Antigenic variation in African trypanosomes: the importance of chromosomal and nuclear context in VSG expression control

Abstract

African trypanosomes are lethal human and animal parasites that use antigenic variation for evasion of host adaptive immunity. To facilitate antigenic variation, trypanosomes dedicate approximately one third of their nuclear genome, including many minichromosomes, and possibly all sub-telomeres, to variant surface glycoprotein (VSG) genes and associated sequences. Antigenic variation requires transcription of a single VSG by RNA polymerase I (Pol-I), with silencing of other VSGs, and periodic switching of the expressed gene, typically via DNA recombination with duplicative translocation of a new VSG to the active site. Thus, telomeric location, epigenetic controls and monoallelic transcription by Pol-I at an extranucleolar site are prominent features of VSGs and their expression, with telomeres, chromatin structure and nuclear organization all making vitally important contributions to monoallelic VSG expression control and switching. We discuss VSG transcription, recombination and replication control within this chromosomal and sub-nuclear context.

Figure 1

The chromosomal context of VSG genes.A. The schematic shows how up to 2000 VSG genes and VSG pseudo-genes occupy the subtelomeres of three different classes of T. brucei chromosomes: megabase, up to 6 Mbp; intermediate, 150–500 kbp; mini, 50–100 kbp. Repetitive sequences found at these loci are thought to be important for monoallelic VSG expression control and for VSG switching via DNA double-strand break (DSB) repair. Telomeric T₂AG₃-repeats cap all T. brucei chromosomes. Megabase chromosomes are diploid with hemizygous sub-telomeres. Bloodstream expression site (BES) promoters, ESAGs and metacyclic expression sites are not shown.B. Mechanisms of antigenic variation; switching from VSG1 to VSG2. Telomere-proximity is thought to render the active VSG locus prone to DSBs. The 70 bp repeats then facilitate gene conversion, replacement of the active VSG with a VSG (or VSG segment) copied from any one of multiple alternative locations. Monoallelic VSG expression is also maintained during an in-situ transcription switch. Only the relevant portions of the BESs are shown.

Figure 2

Nuclear sub-compartments in trypanosomes. A procyclic and bloodstream form nucleus is shown, with the region of heterochromatin (corresponding to ∼ 15% of nuclear volume) in light gray. The nucleolus is shown as a dark gray structure within the nucleus, and in the procyclic form is associated with the procyclin loci (purple). Nuclear pore complexes are shown as double lozenges, and the nuclear lamina as a thin blue lines within the region of heterochromatin. Inactive VSG expression sites are red and located at the nuclear periphery, while the active locus is in green, and is equivalent to the expression site body (ESB). The major changes are that the procyclin loci are downregulated in the bloodstream form and likely come to reside within the heterochromatin, while a single VSG expression site associates with the ESB. While morphologically there may be differences in heterochromatin, evidence is consistent with peripheral chromatin bearing inactive genes in both life cycle stages.